3D Tungsten/SiO 2 Structure Yields Infrared Photonic Bandgap

  • PDF / 953,601 Bytes
  • 1 Pages / 612 x 792 pts (letter) Page_size
  • 2 Downloads / 188 Views

DOWNLOAD

REPORT


spherical particles of CoO and Co 3O 4 that have specific surface areas smaller than 5 m2/g. Standard coin cells were assembled in a helium-filled glove box, which contains a plastic positive-electrode disk and a 1-cm2 Li foil (as the negativeelectrode member), thermally laminated to the metal grid current collectors. A borosilicate glass-fiber sheet, used as a separator, was saturated with a 1 M LiPF6 electrolyte solution in a 1:1 (by weight) dimethyl carbonate/ethylene carbonate. Electrochemical tests were performed using an automatic cycling and data-recording system. A series of different mass ratios (Mr) of CoO/LiCoO2 and Co3O4/ LiCoO2 were attempted to optimize electrochemical performance as a function of particle size. Results showed that the best electrochemical performance is achieved at a particle size of 1 µm. Both systems were able to discharge and recharge for at least 80 cycles at a higher Mr with only minimal capacity decay at room temperature and 100% capacity retention for up to 50 cycles, after an initial irreversible capacity loss during the first cycle. At higher temperature, capacity loss for both Li-ion cells was not significantly affected. To bypass the capacity loss during the first cycle, the researchers replaced the LiCoO2 positive-electrode material with Li1+xMn2O4 as a Li reservoir. By applying this strategy, they were able to compensate the initial irreversible capacity loss. Further calculations also support the researchers’ observations that the performance of the 3D metal oxide electrodes is comparable to commercialized C/LiCoO2 Li-ion cells, at a lower voltage. KINSON C. KAM

3D Tungsten/SiO2 Structure Yields Infrared Photonic Bandgap Three-dimensional (3D) arrangements of tungsten rods embedded in SiO2 have been shown to have photonic bandgaps in the infrared region. S.Y. Lin and J.G. Fleming from Sandia National Laboratories and K.M. Ho and R. Biswas from the Ames Laboratory at Iowa State University have demonstrated this method for the fabrication of 3D tungsten crystals (W-3D) in the May 2 issue of Nature. One of the challenges facing the development of metallic infrared-region photonics is the ability to selectively and precisely deposit or arrange arrays of metallic crystals in three dimensions. The researchers overcame this obstacle by utilizing microfabricated polysilicon/SiO2 structures as crystal molds. The structure reported consisted of a polysilicon/SiO2 photonicbandgap structure with polysilicon rods in a stacking sequence inside the SiO2 that 488

repeated itself every four layers, with a face-centered-tetragonal lattice symmetry. These polysilicon rods were removed by etching in 6 M KOH at 85°C. This effectively left linear voids in SiO2. An adhesion layer of TiN was then deposited, followed by chemical vapor deposition of tungsten, which resulted in W-3D crystals. Thin “keyholes” were located in the center of the tungsten rods. The tungsten layer was removed from the surface by mechanical polishing, and the SiO2 was removed by HF (see Figure). a

w

Figure. Scanning